In the world of modern technology, electronic circuits and circuit elements formed on semiconductor chips are ubiquitous. A semiconductor chip may be found controlling a battery-powered toy, a home stereo or a computerized fuel injection system.
Semiconductor chip technology allows circuits to be miniaturized because microscopic circuit elements and electrical connections between those elements can be formed directly on the chip. This process typically includes successively depositing several layers of different materials on the semiconductor chip that are need to form connections and circuit elements. When necessary, most of the deposited layer or layers can be chemically removed from the chip, leaving material behind only in those places where it will be used to form a circuit element or electrical connection.
Because semiconductor chips are mass-produced, when a defect in the chip is discovered or a circuit element on the chip does not function properly or at all, the cause of the problem must be determined and corrected or it is likely to recur. Accordingly, when a defect or a non-functional circuit element is identified on a semiconductor chip, it must be inspected.
The inspection is carried out by breaking the chip as near the defect or non-functioning circuit element as possible, and then grinding and fine polishing the edge of the chip to expose a cross-section of the defect or the layers of the non-functioning circuit element. The defect or non-functioning circuit element can then be inspected with a microscope to determine the cause of the problem.
The difficulty in the process is polishing the chip with microscopic precision so that the polishing is halted when the desired cross-section of the defect or non-functioning element is exposed. If the polishing is continued too long, the area to be inspected may easily be completely polished away.
Accordingly, during the polishing process, the chip must be periodically inspected with a microscope to determine how much more polishing will expose a cross-section of the defect or non-functioning circuit element for inspection.
Usually the polishing process is accomplished by attaching the semiconductor chip to be polished to a polishing fixture. The polishing fixture then holds the substrate against the surface of a polishing wheel which is rotating in a horizontal plane. The friction between the wheel and the substrate polishes the chip as needed.
In the initial stage of polishing, inspection with an optical microscope may suffice. However, to monitor the end of the polishing process, the chip must be removed from the polishing fixture and viewed with a scanning electron microscope. Once removed from the polishing fixture, the chip is secured to a microscope fixture and inserted in the scanning electron microscope. With the aid of the microscope, it can be determined if the polishing process has been completely and accurately accomplished.
If the polishing is not complete and the cross-section of the defect or non-functioning circuit element is not exposed, the chip must be returned to the polishing fixture for further polishing. It is desirable to be able to fine adjust the rate of polishing to save time. However, it is impossible to resecure the chip to the polishing fixture in precisely the same microscopic relationship that previously existed between the polishing fixture and the chip.
Accordingly, because the precise position of the chip with respect to the polishing fixture has changed, when polishing is resumed, the information obtained from viewing the chip with the scanning electron microscope as to the amount and pattern of further polishing required cannot be implemented precisely. The unknown shift in the position of the chip with respect to the polishing fixture renders the polishing process more imprecise.
Accordingly, there exists a need in the art for a means and method of fine adjustment of the polishing rate and monitoring a semiconductor chip during polishing without altering its position with respect to the polishing fixture.